Jieh Jing is translated from Chinese as "borrowing energy." It's one of the many concepts that are abstractly described as "energies" in the internal arts.
As with each of these "energies," borrowing energy doesn't mean you're sucking the energy out of your opponent literally -- it isn't mystical, it's physical just like all the other skills of Tai Chi, Hsing-I and Bagua.
These three photos show an exercise you can do to practice and demonstrate borrowing energy.
In photo 1, my partner is rushing at me to push me off my ground. He will push hard into my folded arms, using his momentum and weight. I'm establishing the ground path, maintaining peng, and setting up the body structure that I need for good internal mechanics.
In photo 2, he makes contact with his force, or energy. I ground it and give it no place to go but back at him. It actually feels as if it bounces back at him.
In photo 3, you can see him bouncing back slightly. All of the internal mechanics are coming into play here -- ground path, peng jin, dan t'ien rotation, whole-body connection -- do these and it is very difficult for someone to drive you off your ground. Instead, they feel the shock of their force hitting "a brick wall." It can surprise an opponent and give you a window of opportunity to follow up with counters of your own.
Last night I held a free teleconference for members of the online school and a great question was asked -- "If in Tai Chi you're supposed to use an opponent's energy against them, how does borrowing energy fit into that?"
The answer is: you are using the opponent's energy against them, but rather than lead it away or perform roll back, you're bouncing it back.
I'd also like to add a cautionary tale -- beware of any tai chi teacher who demonstrates this and has a student place a finger on him, then causes the student to jump back when the teacher barely moves. More than likely, that is a fraud. Without force coming in, you can't exactly have it bouncing back. And don't expect superhuman feats, such as sending someone flying 20 feet through the air. That only happens when the student really plays along. That said, it's possible when using this in push hands or fighting applications to combine borrowing energy with fa-jing and hit them with enough force to send them back and injure them. I'd hate to be on the receiving end when Chen Xiaowang, Chen Bing or one of those guys used it on me to inflict injury. The above demonstration is a more gentle approach to train this concept with a partner and a way for both people to develop this skill.